1. Technical Field
The present invention relates to a communication system and a time synchronization method, and more particularly to a communication system between devices using a passive optical network (PON), and a time synchronization method.
2. Background Art
As one of public line networks using optical fibers, there is GE-PON standardized by IEEE 802.3ah. With application of Gigabit Ethernet (Registered trademark) which is a technology used in a LAN up to now, the GE-PON enables service provision for two-way transmission of packets of Ethernet (registered trademark) from a station device to a user terminal at the maximum of 1 Gbps. The PON includes an optical line terminal (OLT) and plural optical network units (ONUs), and multiplexes a signal from a terminal (PC, etc.) connected to the ONUs with an optical signal optically (in time division), and transmits the signal to the OLT through the ONU, an optical fiber, an optical splitter, and an optical fiber for the OLT in the stated order. After conducting various signal processing, the OLT conducts communication between a terminal of one ONU, and a terminal of another ONU of the PON or a terminal of an NW. Since the respective ONUs are arbitrarily located, for example, within a range of 0 to 20 km in optical fiber length, transmission delays are different from each other at this rate, resulting in a possibility that the optical signals output from the respective ONUs collide and interfere with each other. Accordingly, as disclosed in Chapter 64 of IEEE 802.3, delays of the output signals from the respective ONUs are adjusted by the aid of a ranging technique as if the respective ONUs were located at the same distance. As a result, the optical signals from the respective ONUs do not interfere with each other on the optical fiber to the OLT.
On the other hand, a cellular phone has a handover function of switching a communication partner to a base station having the highest receiving sensitivity according to travel of a terminal. In an electric communication, for example, a call area of the cellular phone in an extremely small range of, for example, about a dozen m in radius, or a base station in the call area is called “femtocell”. In the related-art base station, a place in which a radio field intensity is weak, and communication is difficult remains in the interior of buildings or an underground. However, when the femtocell is disposed in such a place, a deficit of the call area can be compensated.
In CDMA 2000 and LTE, for the purpose of improving the handover function or a communication quality, time synchronization between the base stations is required, for example, with precision lower than 1 μs. It is general that a GPS signal receiver is disposed in the base station, and the time synchronization is conducted by the receiver. However, it is assumed that the installation of the GPS signal receiver is difficult in the base station of an underground city. Also, it is conceivable that the installation of the GPS signal receiver is economically difficult in an extremely compact base station such as the femtocell. Accordingly, in a system in which the femtocell is installed at home through the above-described PON, there is a need to realize accurate time synchronization over the network.
Presently, as the most important system for time information provision over the network, there is time information distribution by IEEE 1588. The details of this system are disclosed in IEEE 1588, Tutorial. As a first stage, a packet having a time stamp therein is repetitively transmitted from a master side to a slave side to synchronize a clock timer period at the slave side with a clock timer period at the master side. In this stage, because a signal propagation delay between the master and the slave is unclear, even if the clock timer periods can be synchronized with each other, absolute values of the clocks cannot be synchronized with each other. Subsequently, in a second stage, a packet for measuring a round-trip propagation delay (round-trip time between the master and the slave) is transmitted from the slave side to the master side. Upon receiving the packet, the master side returns the packet to the slave side within a specified time. The slave side that has received the packet measures the round-trip propagation delay on the basis of a receive time and a transmit time of the packet, and regards half the above measured round-trip propagation delay as a propagation delay from the master side to the slave side assuming that the propagation delay from the slave side to the master side is substantially equal to the propagation delay from the master side to the slave side. Then, the above-mentioned time stamp value allocated at the master side and the obtained propagation delay from the master side to the slave side are used to synchronize the absolute values of the clock timers at the master side and the slave side with each other.
When the time distribution packet is transmitted through a switch or a router, the packet delay is swayed, and therefore it is desirable to apply this technique on a link-by-link basis. Also, it is possible to enhance the precision with some statistic by obtaining an average value after plural trials are conducted.